Much of the previous epidemiologic research on ovarian cancer has been conducted within the conceptual with conceptual framework of the ovulation and gonadotropin. However, additional mechanisms must be operative to account for epidemiologic findings regarding this disease. In this study, we will address the hypothesis that progesterone reduces risk of epithelial ovarian cancer. We will examine the relation of exogenous progestins administered as a component of hormone replacement therapy (HRT) with disease risk. We will further assess whether sunlight and dietary sources of vitamin D influence risk. The study will contribute to a better understanding of pathogenic mechanisms of epithelial ovarian cancer, and may provide information leading to new means of reducing the occurrence of this disease. We propose to conduct a population-based, case-control study of epithelial ovarian cancer among women aged 35-74 years residing in thirteen counties of Washington State. Cases will be identified through a population-based cancer registry operating as part of the Surveillance, Epidemiology and End Results Program. Controls will be identified through random digit telephone dialing, and will be selected to be similar in age and area of residence to cases. In-person interviews will be conducted and blood samples will be collected. The findings of this study will have appreciable public health importance. Study of the impact of different HRT regimens, particularly estrogen/progestin combinations, on ovarian cancer risk has been deemed an urgent task for research. Recent calls for reappraisals of the risk/benefit ratio of unopposed estrogen and combined estrogen/progestin HRT highlight the need to understand the relation of these medications with ovarian cancer risk, and to incorporate this knowledge into risk/benefit considerations.